1. Field of the Invention
The present invention relates to a CATV tuner. More specifically, the present invention relates to a cable modem tuner incorporated in a cable modem used for enabling high speed data communication at home, utilizing an unused channel of cable television (hereinafter referred to as CATV), and a CATV tuner used for a digital set box (hereinafter referred to as STB) for high speed data communication at home utilizing a different frequency band.
2. Description of the Background Art
In a CATV system, introduction of HFC (Hybrid Fiber/Coax) has been in progress, in which a coaxial cable is kept as a subscriber""s drop wire and the main network is implemented by optical fibers. This system attempts to provide broad-band data communication service of several Mbits/sec at home. Utilizing this system, it is possible to realize high speed data line having the transmission rate of 30 Mbits/sec with the band width of 6 MHz using 64 QAM (Quadrature Amplitude Modulation), which may not be called the state of the art any more. The cable modem is used in this system, and realizes high speed data communication of 4 Mbits/sec to 27 Mbits/sec, utilizing an unused channel of CATV.
FIG. 11 is a block diagram of a conventional cable modem tuner. An up signal transmitted from the cable modem tuner to a CATV station, not shown, has the frequency of 5 MHz to 42 MHz, and a down signal transmitted from the CATV station to the cable modem tuner has the frequency of 54 MHz to 860 MHz, and transmitted to a cable network through a CATV input terminal 11 of the tuner. The up signal transmitted from the cable modem is received by a data receiver of the CATV station (system operator), and enters a computer of a center. In the cable modem, a data signal subjected to quadrature phase shift keying from a QPSK transmitter, not shown, is input to a data terminal 10, as the up signal. The data signal is transmitted through an upstream circuit 9 and an input terminal 11, to the CATV station.
The down signal is passed through an HPF (High Pass Filter) 1 as an IF (Intermediate Frequency) filter having an attenuation range of 5 to 42 MHz and a passband of not lower than 54 MHz and to a buffer amplifier 35 to be supplied to various circuits of the succeeding stages.
The circuits of the succeeding stages provide receiving circuits for UHF band (B3 band) having the frequency of 470 to 860 MHz, VHF High band (132 band) of 170 to 470 MHz and VHF Low band (B1 band) of 54 to 170 MHz, respectively. Band division is not limited thereto.
Further, the cable modem tuner includes, in addition to the receiving circuits described above, IF amplifying circuits 19 and 21, an SAW filter 20, an IF output terminal 12 and a PLL channel selection circuit 27.
The receiving circuits for the B1 to B3 bands described above respectively include input switching circuits 200, 140 and 220; UHF high frequency amplification input tuning circuits 300, VHF HIGH BAND high frequency amplification input tuning circuit 150 and VHF LOW BAND high frequency amplification input tuning circuit 230; a UHF high frequency amplifier 4, a VHF HIGH BAND high frequency amplifier 16 and a VHF LOW BAND high frequency amplifier 24; a UHF high frequency amplification output tuning circuit 50, VHF HIGH BAND high frequency amplification output tuning circuit 170 and VHF LOW BAND high frequency amplification output tuning circuit 250; a UHF mixing circuit 6, a VHF HIGH BAND mixing circuit 18 and a VHF LOW BAND mixing circuit 26; and a UHF oscillating circuit 7, a VHF HIGH BAND oscillating circuit 13 and a VHF LOW BAND oscillating circuit 8, corresponding to the mixing circuits, respectively.
Switching method using a switching diode, or a method using a filter for band splitting is applied to the input switching circuits 200, 140 and 220.
Generally, a dual gate type MOSFET device is used for the high frequency amplifiers 4, 16 and 24. An AGC (Automatic Gain Control) voltage from an AGC terminal 36 is input to the gate electrode of the device, and therefore the gain in these amplifiers is controlled by the AGC voltage.
Input switching circuits 200, 140 and 220 receive as inputs the signals of B1 to B3 bands, and selectively outputs the received signals of prescribed frequency bands only.
High frequency amplification input tuning circuits 300, 150 and 230 tune the received signals selectively output from input switching circuits 200, 140 and 220 to respective desired frequencies (frequencies of the desired channels) using a tuning coil or the like, in respective bands.
High frequency amplifiers 4, 16 and 24 receive the output signals from high frequency amplification input tuning circuits 300, 150 and 230, amplify these signals so as to prevent degradation of SN ratio such as signal distortion, using the voltage level of AGC terminal 36 receiving the AGC voltage as a reference, and output the resulting signals. The RF (high frequency) AGC voltage supplied to AGC terminal 36 is supplied to the gate electrode of the dual gate MOSFET in each of the high frequency amplifiers 4, 16 and 24, and therefore the dual gate MOSFET operates such that the power gain of the high frequency amplifier attains the full gain when the input signal level is higher than 60 dBxcexc, and operates so that the output level of the tuner is always kept at a constant level when the input signal level is not higher than 60 dBxcexc, so that degradation of SN ratio such as distortion, of the signal can be prevented.
High frequency amplification output tuning circuits 50, 170 and 250 tune the output signals from high frequency amplifiers 4, 16 and 24 to desired frequencies by using a tuning coil or like in respective bands, and provide the resulting signals.
Local oscillating circuits 7, 13 and 8 oscillate to provide prescribed intermediate frequencies corresponding to respective bands, and mixing circuits 6, 16 and 26 convert the signals output from high frequency amplification output tuning circuits 50, 170 and 250 to desired intermediate frequency signals by using the oscillation signals from the corresponding local oscillating circuits, and therefore, local oscillating circuits 7, 13 and 18 together with the mixing circuits 6, 18 and 26 form frequency converting circuits for respect bands.
Thereafter, the output signals of the receiving circuits are amplified to prescribed levels by an IF amplifying circuit 19, frequency-converted to a prescribed level by SAW filter and IF amplifying circuit 21, and output through IF output terminal 12.
In operation, the down signal passes through HPF1 and applied to input switching circuits 200, 140 and 220. Therefore, among the three receiving circuits, only that receiving circuit of which operational frequency corresponds to the frequency of the down signal operates, and other receiving circuits do not operate. The operations of the receiving circuits are common.
The receiving circuit of each band will be described in the following.
CATV signal is passed through input switching circuits 200, 140 and 220 as well as high frequency amplification input tuning circuits 300, 150 and 230, amplified by high frequency amplifiers 4, 16 and 24, and provided as received signals through high frequency amplification output tuning circuits 50, 170 and 250.
Thereafter, the received signals are passed through mixing circuits 6, 18 and 26 as well as local oscillating circuits 7, 13 and 8 whereby the signals are converted to desired intermediate frequency signals, and subjected to LOW IF conversion by IF amplifying circuits 19 and 21 and SAW filter 20, and provided at output terminal 12.
The above described series of operations are implemented as a channel selection data is transmitted from a CPU, not shown, to PLL channel selection circuit 27 so that the channel is selected accordingly and, at the same time, the input switching circuit for band switching operates in accordance with the band characteristic so that the power supply to respective bands is switched.
A cable modem tuner having a similar structure is also disclosed in Japanese Patent Laying-Open No. 10-304261.
The conventional cable modem tuner described above operates such that it is always kept in a stand-by state for reception. Therefore, low power consumption is necessary. In the double conversion type cable modem tuner described above, power consumption in the stand-by state is 0.7 to 1W, which is relatively large as compared with the power consumption in operation.
More specifically, in the conventional cable modem tuner, the high frequency amplifying circuits 4, 16 and 24 operate independent from each other, and therefore a current for switching operations of these circuits is necessary. Further, when a multiwave signal of the CATV is received at the buffer amplifier 35, the received signal is prone to distortion. In order to solve this problem, it is necessary to supply large current to the device of the buffer amplifier 35.
Further, the cable modem tuner, which is a CATV receiver, receives the multiwave signal in common. Therefore, at least 6 dB of an input return loss is necessary for the entire reception band. Therefore, buffer amplifier 35 is inserted to the input circuit of the conventional cable modem tuner, to improve the input return loss. Further, in the above described conventional cable modem tuner, AGC is realized in the stage of high frequency amplifiers 4, 16 and 24. Such a system is susceptible to intermodulation distortion and mixed modulation distortion.
More specifically, AGC is realized at high frequency amplifiers 4, 16 and 24 of FIG. 11, and the high frequency amplifiers are generally implemented by dual gate type MOSFETs, and therefore linearity in AGC operation is not satisfactory. Further, as the signal level is amplified by buffer amplifier 35, the signals to be applied to high frequency amplifiers 4, 16 and 24 of the succeeding stages come to have high signal level, so that intermodulation distortion and mixed modulation distortion are more likely when the high frequency signal components are amplified.
Further, in the above described conventional cable modem tuner, because of the nature of the dual gate type MOSFETs used for high frequency amplifiers 4, 16 and 24, high frequency parameter component at the input/output fluctuates by the AGC operation, resulting in waveform distortion (waveform fluctuation), which leads to higher possibility of transmission distortion.
Further, as the device characteristics of the high frequency amplifiers 4, 16 and 24 have the above described disadvantages, signal transmission distortion (amplitude distortion) resulting from AGC is highly likely, of which improvement is very difficult.
Further, as the high frequency amplifying circuits 4, 16 and 24 of the conventional cable modem tuner are provided for respective bands, the number of circuit components is considerably large, which is not preferable in view of economy.
In FIG. 11, a cable modem tuner is shown. Recently, a CATV tuner generally referred to as a digital set top box (hereinafter referred to as a STB) has been proposed. The cable modem allows display of the down data signal transmitted from the CATV station to be displayed on a television monitor. The STB branches QPSK modulated down data signal transmitted from the CATV station from the tuner, and signals are processed by a CPU to be output to a personal computer.
Therefore, while the unused channel of CATV in a band between 54 MHz to 860 MHz is used for transmitting the down data signal as described above, different frequency band of 70 MHz to 130 MHz is used in the STB.
FIG. 12 is a schematic block diagram of the STB which includes a branching circuit 37 for branching the down data signal between HPF1 and buffer amplifier 35, and the branched down data signal is output to an OOB (Out Of Band) terminal 38. The OOB terminal outputs the branched data to the CPU. Except this point, the structure is the same as that of FIG. 11.
In the STB shown in FIG. 11, the up signal of the CATV signal has the frequency of 5 MHz to 42 MHz, and the down signal has the frequency of 54 MHz to 860 MHz, and the signals are connected through input terminal 11 to the cable network. The up signal transmitted from the STB is received by a data receiver of the CATV station and input to a computer of the center.
In the STB, a QPSK data signal from a QPSK transmitter (not shown) is provided at data terminal 15 as the up signal. The data signal is input to the STB through the CATV network by the computer at the center, processed by the CPU (not shown) in the STB, and applied to a QPSK modulator. Except this point, the operation is the same as that in the cable modem tuner shown in FIG. 11, and the STB also has the same problems as the cable modem tuner described above.
Therefore, an object of the present invention is to provide a CATV tuner of which power consumption is saved.
Another object of the present invention is to provide a CATV tuner of which signal distortion can be suppressed.
Briefly described, the present invention provides a CATV tuner including an upstream circuit for transmitting a data signal to a CATV station, a high pass filter for receiving, while removing a data signal, multiwave down signal from the CATV station, and a receiving unit for receiving the down signal provided through the high pass filter, wherein the receiving unit includes a gain control circuit receiving the down signal, attenuating the same with a prescribed gain and thereafter amplifying and outputting the resulting signal, a high frequency amplifying circuit receiving the output signal from the gain control circuit and extracting a frequency signals of respective ranges of different frequency bands, a frequency converting circuit converting the signal output from the high frequency amplifying circuit to a prescribed intermediate frequency signal for each range and outputting the resulting signal, and an intermediate frequency amplifying circuit amplifying the output signal from the frequency converting circuit and providing the resulting signal.
Therefore, according to the present invention, the down signal is attenuated by a prescribed gain at the input unit of the receiving unit, amplified, desired frequency signal is extracted for each range by the high frequency amplifying circuit and amplified, thereafter converted to a desired intermediate frequency signal by the frequency converting circuit for each range, and amplified by the intermediate frequency amplifying circuit to be output. Therefore, the down signal is attenuated by the prescribed gain before amplification at the high frequency amplifying circuit at the input unit. Therefore, even when the down signal as the multiwave signal is received with high input level, generation of a signal distortion can be suppressed. Further, the level of the signal input to the succeeding circuit portion for amplification can be made lower, and therefore current consumption in the high frequency amplifying circuit can be reduced.
In a preferred embodiment of the present invention, a down data signal having a different band from the multiwave down signal from the CATV station is input through the cable to the receiving unit, and the receiving unit includes a branching circuit for branching and outputting the down data signal.
Therefore, in the preferred embodiment, by the branched down data signal, data communication with the CATV station can be established, regardless of the tuner unit.
Further, in a more preferred embodiment, the high frequency amplifying circuit includes an input selecting circuit receiving an output signal from the gain control circuit and selectively outputting the signal to a plurality of ranges dependent on the frequency band, a high frequency amplification input tuning circuit provided for each of the plurality of ranges, receiving the signal for each range selected by the input selecting circuit, tuning the received signal to a desired frequency and outputting the result, a high frequency amplifying circuit provided commonly for the plurality of ranges, amplifying output signals from respective high frequency amplification input tuning circuits and outputting a result, an output selecting circuit receiving the output signal from the high frequency amplifying circuit and selectively outputting signals of a plurality of ranges, and a high frequency amplification output tuning circuit provided for each of the plurality of ranges, receiving the signals of respective ranges selectively output from the output selecting circuit, tuning the signals to desired frequencies and outputting the resulting signals.
In this embodiment, it is unnecessary to provide a high frequency amplifying circuit for each of the plurality of ranges as in the prior art but only one high frequency amplifying circuit is sufficient. Therefore, the current consumption can be reduced, and the number of circuit components constituting the tuner can be reduced, which leads to reduced cost.
More preferably, the high frequency amplifying circuit includes an input selecting circuit receiving an output signal from the input unit and selectively outputting to a plurality of ranges dependent on the frequency band, at least two high frequency filter circuits provided for the plurality of ranges, receiving signals of at least two ranges selected by the input selecting circuit and cutting off the frequencies other than the desired frequency, a high frequency amplifying circuit provided commonly to the plurality of ranges, for amplifying the output signals from the filter circuit and outputting a result, an output selecting circuit receiving the output signal from the high frequency amplifying circuit and selectively outputting signals of at least two ranges, and a high frequency amplification output selecting circuit provided for each of the plurality of ranges, receiving the signals of at least two ranges output from the output selecting circuit, and tuning the signals to the desired frequencies of respective ranges and outputting the result.
Therefore, while the high frequency amplification input tuning circuit and the high frequency amplifying circuit have been provided for each of the plurality of ranges, what is necessary is only to provide at least two high frequency filter circuits for two ranges and one high frequency amplifying circuit. Therefore, the current consumption can be reduced and the number of circuit components can be reduced, whereby the overall cost is reduced.
Further, the gain control circuit includes an attenuating circuit attenuating the down signal with a prescribed gain and providing the result, and a buffer amplifying circuit receiving an output signal from the attenuating circuit, amplifying the same over a broad-band and outputting the result.
Therefore, in the present embodiment, the down signal can be amplified over a broad-band without causing any distortion in the buffer amplifying circuit, and hence the signal distortion at the time of signal reception in the tuner can be improved.
Further, the prescribed gain can be variably set based on the input signal level at the high frequency amplifying unit.
Therefore, the amount of attenuation of the signal by the gain control circuit can be determined based on the level of the signal input to the high frequency amplifying circuit of the succeeding stage. Therefore, the level of the down signal to be applied to the high frequency amplifying circuit can be set to such a level that allows stable operation of the high frequency amplifying circuit not causing signal distortion. As a result, even when the down signal is input at high level to the high frequency amplifying circuit, distortion at the time of signal transmission can be avoided.
Further, at least one of the input selecting circuit and the output selecting circuit includes a plurality of switching elements operating based on the input signal level, and a plurality of inductor elements of which switching is controlled in accordance with the operation of the plurality of switching elements. The input signal is selectively output to the plurality of ranges by the switching control of the plurality of inductor elements in accordance with the operation of the plurality of switching elements.
Therefore, in the present embodiment, the circuit structure necessary for switching between ranges can be simplified by the switching of the inductor elements in accordance with the operation of the switching diode element. Therefore, the number of components can be reduced, leading to lower cost, and the current consumption can be reduced.
Further, the high frequency filter circuit includes a combination circuit of a high pass filter and a low pass filter of which cut off frequencies are variable, and the high frequency amplifying circuit includes a bipolar transistor or a dual gate transistor.